A quadrature local oscillator (LO) signal is an essential element in the direct conversion and low intermediate frequency (IF) transceivers for wireless communication technology, e.g., a Quadrature Amplitude Modulation (QAM) system. QAM is a modulation scheme for communication signals. It conveys two signals, an in-phase signal (I), and a quadrature signal (Q), with amplitude modulation of the two carrier waves. These two carrier waves for I and Q are out of phase with each other by 90°.
The quadrature signals (I and Q) can be generated by different methods. One conventional differential oscillator utilizing a divide-by-2 circuit that works at twice the desired frequency, has poor quadrature accuracy, and may have large power consumption. Another differential oscillator utilizing a poly-phase filter is subject to process variations, and a buffer amplifier is needed to compensate for the loss of the poly-phase filter.
For quadrature oscillators coupling two differential oscillators using active devices, e.g., transistors, such conventional quadrature oscillators have phase noise degradation due to added noise sources, extra power consumption, less voltage headroom, and/or LC resonant frequency effects. Also, conventional coupling between two differential oscillators using transformers, incurs large chip area consumption.